The present invention relates to a method for measuring the flow of fluids, herein after referred to as xe2x80x9cflow measurementxe2x80x9d. It should, however, be understood that the term xe2x80x9cflow meausrementxe2x80x9d as used throughout this specification means not only a measurement of the flow-velocity of a gas, such-as air, fuel gas, etc., or a liquid, such as water, liquefied gas, etc., but also a topological visualization of the distribution of such gas or liquid.
The particles heretofore used as tracer particles in optical flow measurements are porous particles made of SiO2, TiO2, SiC or the like which are obtainable by a coprecipitation process or from a natural material such as the mineral ore. These particles generally have a mean particle diameter of about 0.5 to 150 xcexcm.
In a measurement of the flow velocity using a laser device such as a laser Doppler velocimeter, a phase Doppler velocimeter or the like, tracer particles somewhere between 0.5 and 10 xcexcm in mean diameter, in particular, have so far been employed.
In technologies involving a visualization of a flowing fluid by photographing the distribution of tracer particles in the fluid with the aid of an instantaneous, powerful light source, such as a flash-light or a pulse laser, and a determination of the flow pattern from the resulting picture, particles somewhere between about 5 xcexcm and about 150 xcexcm in mean diameter are generally employed.
Electron microphotographs of the representative tracer particles which are conventionally employed are presented in FIGS. 3 through 14; viz. white carbon in FIGS. 3 and 4, TiO2 in FIGS. 5 and 6, talc in FIGS. 7 and 8, TiO2-talc in FIGS. 9 and 10, particles from kanto loam, and white alumina in FIGS. 13 and 14.
However, as apparent from these microphotographs, the conventional tracer particles have the following drawbacks, 1) through 5), which amplify the measurement error.
1) Because the tracer particles are morphologically not uniform, the sectional area of scattered light to be detected varies according to the real-time orientation of each particle.
2) Because the particle size distribution is broad and the sectional area of light scattering varies with different individual particles, the comparatively large particles scatter light in two or more fringe at a time.
3) Because the apparent specific gravity of the particulate tracer differs markedly from that of the fluid to be measured, the particles do not faithfully follow the on-going flow of,the fluid.
4) Because the particle size distribution is broad and the apparent specific gravity also has a distribution, the particles follow the fluid flow with varying efficiencies to prevent accurate quantitation of the flow measurement.
5) Because the surface of the particle is irregular, the individual particles tend to be concatenated with each other to increase the effective particle size.
The technique used generally for launching tracer particles into a fluid comprises either extruding tracer particles from a screw feeder and driving them into the body of the fluid with the aid of an air current or suspending tracer particles in a solvent and ejecting the suspension in a mist form using an ultrasonic humidifier. In any of the above methods, the rate of feed of the tracer particles is not constant so that the accuracy of flow measurement is inevitably sacrificed.
It is the object of the present invention to overcome the above-mentioned drawbacks and provide a method of flow measurement with improved accuracy.
The method of flow measurement according to the invention comprises measuring the flow of a fluid using an optical instrument and a porous particulate ceramic tracer, the diameter of which is 0.5 to 150 xcexcm.
In another aspect, the method of flow measurement according to the invention comprises feeding a non-agglomerating particulate tracer to an optical instrument, such as a laser device, from a measuring wheel particle feeder.
The method of flow measurement according to the invention comprises measuring the flow of a fluid using an optical instrument and a porous particulate ceramic tracer, said porous particulate ceramic tracer consisting of spherical particles having a diameter of 0.5 to 150 xcexcm. Particularly in the method of measuring the flow velocity using a laser instrument such as a laser Doppler velocimeter, spherical ceramic particles having a diameter of 0.5 to 10 xcexcm are preferred from the viewpoint of relation with fringe. A more satisfactory spherical particle diameter range is 1.5 to 2.5 xcexcm. In flow measurement which involves photographing, the use of spherical particles having a diameter of 5 to 150 xcexcm is preferred from the viewpoint of detecting light and flowing the fluid flow. A more satisfactory particle diameter range is 30 to 100 xcexcm.
When the tracer particles for use in flow measurement with an optical instrument are spherical as in the invention, the sectional area of scattered light to be detected by a photosensor or the like is constant regardless of the orientation of particles at the moment of detection. Moreover, because such particles have no surface irregularities that may cause concatenation, it does not happen that two or more tracer particles flow as concatenated through the body of the fluid. Therefore, the accuracy of flow measurement is improved.
Where the fluid to be measured is a gas, said tracer particles are preferably of hollow structure.
When the tracer particles are hollow, the specific gravity of the particles is so low that even if the particle size is not critically uniform, they may. readily follow the gas flow. Therefore, the accuracy of gas flow measurement is improved. The improved accuracy of measurement afforded by such hollow spherical particles over that attainable with solid spherical particles is more remarkable when the flow rate of the fluid is high.
The shell thickness of such hollow spherical particles is not so critical but is preferably in the range of one-third to one-tenth of the diameter of the particle. If the shell thickness is less than one-tenth of the particle diameter, the particles tend to be collapsed in use. Conversely when the shell is thicker than one-third of the particle diameter, the advantage of the hollow structure will not be fully realized.
Where the fluid to be measured is a liquid, said tracer is preferably a porous particulate ceramic tracer having closed pores with a porosity of not less than 0.1 cm3/g.
When the tracer particles have closed pores with a porosity of not less than 0.1 cm1g, the specific gravity of the tracer particles can be changed so as to minimize the differential from the specific gravity of the fluid to be measured, thereby making it easier for the particles to follow the dynamics of the fluid. In this manner, the accuracy of flow measurement can be further improved.
Where the fluid to be measured is a liquid, tracer particles coated with a metal are used with advantage.
When such metal-clad porous spherical particles are used for the flow measurement of a liquid, the intensity of reflected light is greater than it is the case when bare particles are employed so that the accuracy of flow measurement is improved. However, since such metal-clad particles are higher in specific gravity and expensive, they are preferably used where the conditions of measurement specifically call for the use of such particles.
Particularly preferred are metal-clad porous ceramic tracer particles having closed pores with a porosity of not less than 0.1 cm3/g. Application of a metal cladding increases the specific gravity of particles as mentioned above but the adverse effect of increased specific gravity can be minimized by using porous ceramic particles having closed pores with a porosity of not less than 0.1 cm3/g.
For application of a metal cladding, any of the electroless plating, electrolytic plating, CVD, vapor deposition and other techniques can be utilized but the electroless plating process is preferred in that a uniform cladding can-be easily obtained.
The cladding metal includes, among others, Ni, Pt, Co, Cr, etc. but nickel is preferred in that a quality cladding can be easily obtained by electroless plating and that the resultant cladding is comparatively high in chemical resistance.
The thickness of the metal cladding is not critical but is preferably within the range of 0.05 to 5 xcexcm. If the cladding thickness is less than 0.05 xcexcm, the effect of increased reflectance is hardly obtained. If the cladding is over 5 xcexcm in thickness, the proportion of the metal in the-whole particle is too large so that the bulk specific gravity of the tracer is increased.
The starting material for said particulate tracer or for the ceramic part of said metal-clad particulate tracer is not limited in variety only if it is chemically stable. Thus, the starting material-can be selected from among, for example, alkaline earth metal carbonates such as calcium carbonate, barium carbonate, etc., alkaline earth metal silicates such as calcium silicate, magnesium silicate, etc.; and metal oxides such as silica (SiO2), iron oxide, alumina, copper oxide and so on. Among these materials, SiO2 is particularly desirable in that it is commercially available at a low price and resistant to heat. When the heat resistance of the ceramic material is high, particles prepared therefrom can be effectively used without the risk of breakdown even in high-temperature fluids.
The size distribution-of tracer particles is preferably as narrow as possible but when not less than 70% of the particles have diameters within the range of xc2x150% of the mean particle diameter, there is obtained a substantially uniform sectional area of scattered light. Moreover, the kinetics of tracer particles in the fluid body, that is to say the pattern of following the fluid flow, are then rendered substantially uniform.
The tracer particles of the invention can be applied to the measurement of fluids flowing at high speeds. Thus, in the conventional flow measurement using a laser Doppler device, an attempt to increase the sample data rate (the number of data generated per unit time) by increasing the flow rate of the fluid and, hence, the number of tracer particles passing through the fringe per unit time resulted in a decrease in the mean effective data rate, which is a representative indicator of measurement accuracy, thus making it difficult to achieve an accurate measurement of a high-velocity fluid. In accordance with the present invention, the mean effective data rate is high even at a high sample data rate so that the method can be effectively applied to the measurement of fluids flowing at high speeds.
Furthermore, in the conventional flow measurement, the concentration of tracer particles cannot be increased over a certain limit because an increased feed of tracer particles for generating a larger number of data per unit time should adversely affect the mean effective data rate. However, in the method of the invention, increasing the rate of feed of tracer particles for increasing the sample data rate does not sacrifice the mean effective data rate, with the result that the desired measurement can be performed with an increased tracer concentration.
The particulate tracer or the ceramic core of the metal-clad particulate tracer can be easily manufactured at low cost by the reversed micelle technology which provides spherical or hollow spherical porous tracer particles.
In this connection, when an aqueous solution of the precursor for the tracer material is extruded from .a porous glass or polymer membrane having substantially uniform pores in an organic solvent, there can be obtained uniform particles with a narrow size distribution and such particles are well suited for use as the tracer particles or the core of metal-clad tracer particles.
The above-mentioned porous glass or polymer membrane may be any of the known membranes such as the membrane obtainable by subjecting borosilicate glass to phase separation and washing the product with a pickling acid solution, the membrane obtainable by mixing a silica sol with a water-soluble organic polymeric material, subjecting the mixture to phase separation at polymerization and rinsing the product, and the membrane obtainable by a technology involving irradiation with laser light to give perforations of substantially uniform diameter.
The tracer particles can be advantageously fed to the laser instrument by means of a measuring wheel particle feeder.
When the tracer particles-are fed from the measuring wheel particle feeder, the particles can be delivered quantitatively so that the accuracy of velocity measurement or photographic distribution measurement is further improved. Moreover, in the conventional method for obtaining of the high measurement accuracy, it is essential to recalibrate the instrument after each measurement cycle for minimizing the measurement error. This operation is eliminated by use of the measuring wheel particle feeder so that as many more measurements can be performed within a given time period.
The construction of the measuring wheel particle feeder and the mechanism of feed are described below, referring to FIGS. 15 and 16. As illustrated, a feeder body 101 is internally provided with a disk 102 which is driven by a motor not shown. The top surface of this disk 102 is provided with a circumferential groove 103.
The reference numeral 104 indicates a hopper which is filled with a particulate tracer F. The hopper 104 has a lower portion 104a which is tapered towards the discharge end of the hopper and the lowest part 104b thereof is open in the form of an orifice 104c immediately over the groove 103, so that the particulate tracer F in the hopper 104 may flow through the orifice 104c into the circumferential groove 103.
The reference numeral 107 indicates a blow nozzle made of plate material. This blow nozzle 107 is configured as a sector in plan view and has a recess 109 having a tapered lateral surface 108 in a substantial center thereof. This recess 109 is centrally provided with an orifice extending in the direction of the thickness for passage of tracer particles (FIG. 16).
The reference numeral 105 indicates a particle duct which runs through a casing 106 of the feeder body 101 and through which the inside of the feeder body 101 is made communicable with the outside thereof. This particle duct 105 is attached to the top of the blow nozzle 107 in such a manner that its inward end 105a covers said recess 109 to establish communication with said particle duct 110.
The atmospheric pressure within the feeder body 101 is maintained at a level higher than the external atmospheric pressure. Because of this pressure gradient, the air flows into the circumferential groove 103 adjacent said blow nozzle 107 at point X beneath the blow nozzle 107. The air then flows out through a particle passageway 110, said recess 109 and said particle duct 105. The arrowmarks in FIG. 16 indicate the flow of air.
As the particles F are carried by such an air flow, they are successfully metered out from the feeder body 101 into the body of the fluid to be measured.
In a second aspect, the invention provides a method of flow measurement using an optical instrument and a particulate tracer material, wherein a non-agglomerating particulate tracer is fed to the laser or other optical instrument with such a measuring wheel particle feeder.
When a non-agglomerating particular tracer material is fed with the measuring wheel particle feeder for optical instrument, the feed rate can be critically controlled even when the tracer has a large particle size distribution and is morphologically divergent as it is the case with the conventional tracer particles. Thus, the conventional non-agglomerating tracer particles are generally large in particle size and high in bulk specific gravity so that they cannot faithfully follow the fluid flow but when this measuring wheel particle feeder is employed, a better tracking performance can be obtained for enhanced measuring efficiency under conditions of high flow rate and least turbulence.